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Coded image data
5
3
2
3
QAM-modulated
2
image data
0
2
3
3
2
5
Fig. 5-46 Example of QAM Method (image)
• Modulation speed of QAM method
The QAM method modulates the carrier wave every 1/2400 seconds. In
other words, there are 2400 phase changes every second. Accordingly, the
modulation speed of this QAM method is 2400 baud, and the transmission
speed becomes 9600 bps.
•
ITU-T
recommendation V.29
ITU-T recognizes QAM methods that transmit data at a speed of 9600 bps,
and related standards are called ITU-T recommendation V.29.
(2) 7200 bps QAM methods
The 7200 bps QAM methods technically is the same as the 9600 bps QAM
methods. The same phase changes are used, and the modulation speed is
also the same. However, the 7200 bps QAM methods differs in respect that
the carrier wave does not carry any information. In other words, the initial
bit of the quad bit is fixed to "0" and is a so-called unwanted bit. The
remaining bits are used to change the phase, and the modulation speed is
2400 baud. For this reason, the data transmission speed becomes 7200 bps.
7200 bps QAM systems are also called ITU-T recommendation V.29 in the
same way as 9600 bps QAM methods.
G3 FACSIMILE COMMUNICATIONS
Modulation speed (baud rate) = 2400 baud
Transmission speed = 9600 bps
Time
Modulated as amplitude
Modulated as phase shift
1/2400 sec
0 0 1 0
1 1 1 0
0 0 1 1
"Small"
"Large"
"Small"
amplitude
amplitude
amplitude
waveform
waveform
waveform
advanced
advanced
advanced
from
from
from
waveform
waveform
waveform
on left by
on left by
on left by
90
225
135
1 0 0 1
1 1 0 1
"Large"
amplitude
waveform
of same
phase
waveform
on left
5–53
1
2
3
4
5
6
ït
çi
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